The standard "closed barrel cage" as specified in American Petroleum Institute publication, Specification 11AX For Subsurface Sucker Rod Pumps and Fittings 9th edition, 1989 (A.P.I. Spec. 11AX), and designated as C14-15, C14-20-125, C14-20, C14-25, and C14-30, has been the only cage offered by the major pump manufacturers for use in standard, stationary barrel, top or bottom anchored, rod insert pumps (i.e.--A.P.I. Spec. 11AX pump designations: RSA, RSB, RHA, RHB, RWA, and RWB). These cages have proven to be economical, relatively free flowing, and durable under most average pumping conditions. And they have been modified, with some degree of success, to enhance durability and performance in more demanding conditions. However, closed barrel cages have some inherent design deficiencies that cause problems with pumping systems which have plagued their users for years.
The first and foremost design limitation is in how they position the lower stationary or standing ball and seat valve. All current designs of these cages are quite long, which is necessary to give the closed design adequate fluid passage, and to allow for the long, external thread on top (A.P.I. Spec. 11AX "C11" pin thread) which screws into the bottom of the pump barrel. This results in a great deal of space or unswept volume between the lower standing valve (i.e.--the suction) and the upper travelling valve (i.e.--the discharge) when they are closest together at the bottom of the plunger down-stroke. This unswept volume presents no major problem if the fluid being pumped is all or mostly all liquid because liquids are nearly incompressible. Crude oil, however, usually contains dissolved natural gas, some of which separates from the liquid when subjected to the drop in pressure caused by the up-stroke of the pump plunger. In addition, free natural gas is usually found in the formation and will inevitably gravitate to the pump suction. If enough of the swept volume of the pump is filled with gas instead of liquid, then a condition known as "gas locking" can occur. Gas lock occurs when the gas in the pumping chamber is not compressed to a sufficiently high pressure, during plunger down-stroke, to overcome the hydrostatic pressure being exerted on the top of the closed travelling valve check ball. This hydrostatic pressure is due to the weight of the fluid column above the pump inside the production tubing. This failure of the travelling valve to open prevents the pump from discharging the fluid inside the pumping chamber.
A further limitation of the closed barrel cage design is that its internal passages are more intricate and therefore more restrictive than their open cage counterparts. This is a considerable disadvantage when used in the standing (suction) valve position because the potential pressure differential is not as great as is possible in the travelling (discharge) valve position. This restrictive design also tends to aggravate any potential problem with gas locking because they are more likely to cause the dissolved natural gas to separate, much like the effect of agitating carbonated water. Closed barrel cages also have more of a tendency to become clogged by foreign matter from the formation which can further restrict flow and cause even more undesirable gas separation.
There have been attempts to remedy some of the problems associated with closed barrel cages such as reducing the cage volume, using various types of inserts to guide and contain the ball with much less restriction to flow, and some have even modified other parts of the pump, or used mechanical devices to force the operation of the valves. However, seemingly none of the previous efforts have been in wide acceptance by the marketplace as a universal and foolproof solution to any of the before mentioned problems, especially gas lock. In fact, the closest thing to a cure for the problems associated with the use of closed barrel cages probably pre-dates their invention--the long defunct McGregor Working Barrel Pump Co. of Bradford, Pa. produced a 15/8" bore, stationary, rod pump which had a seating mandrel which screwed into the bottom of the pump barrel tube, and just above the barrel threads on the mandrel was another threaded section, smaller in diameter, which screwed onto an open type cage (similar to A.P.I. Spec. 11AX designation C17-150) which contained a "rib type" ball and seat check valve. This design placed the standing valve up inside the barrel about two or three inches (instead of two inches below the barrel like a closed barrel cage does) and yielded a very small unswept volume with a very high compression ratio. The drawbacks were that the travelling valve was placed on top of the plunger instead of below it, and that the pump required many special "McGregor only" parts rather than A.P.I. types which caused its eventual demise.